Euglena gracilis was grown in the dark for 12 days. Subsequent incubation with sodium octanoate- l- 14 C in the dark for 6 hours resulted in a rather specific incorporation of radioactivity into the neutral lipids (primarily wax esters). Upon illumination of these cells in an identical medium without radiocarbon, the radioactivities in the neutral lipids decreased strongly, due to a decrease of the labeled wax acids (mainly 1 4:0) and wax alcohols (mainly C 14 ). The radioactivities in phosphatidyl choline and phosphatidyl ethanolamine increased during the first 24 hours of illumination and thereafter decreased. This was caused by the initial increase and subsequent decrease of the labeled 14:0 and 16:0 acids in phosphatidyl choline and of the 14:0, 16:0 , and 18:1 acids in phosphatidyl ethanolamine. The chloroplast lipids (sulfolipid, phosphatidyl glycerol, monogalactosyl diglyceride, and digalactosyl diglyceride) exhibited a steady increase in radiocarbon content. This was due to an increase of label in the 16:0 fatty acid of the sulfolipid, in the 16:0 and trans3-16:1 fatty acids of the phosphatidyl glycerol, and in the saturated and unsaturated C 16 and C 18 fatty acids of the monogalactosyl and digalactosyl diglycerides. The labeled fatty acids of the above phospho- and glycolipids had comparably high specific radioactivities. Incorporation of radiocarbon into the last two carbon atoms of their methyl ends, however, was low. The glycerol and sugar moieties of the individual lipids incorporated relatively little radiocarbon. It is concluded that in Euglena gracilis the biosynthesis of long chain fatty acids is associated with specific lipids. Upon illumination, oxidative breakdown of the neutral lipids as well as transfers of fatty acids from the neutral lipids via phosphatidyl choline and phosphatidyl ethanolamine to the chloroplast lipids seem to be induced. The lipids involved appear to function as parts of a “lipid-bridge” for the acyl transfers.